Method and apparatus for transmitting and receiving power save-polling frame and response frame in wireless LAN system

ABSTRACT

A method and apparatus for transmitting and receiving a power save-polling (PS-Poll) frame and response frame in a wireless LAN (WLAN) system are disclosed. The method for transmitting a frame by a station (STA) in a wireless LAN (WLAN) system includes awakening at a predetermined time, and transmitting a Power Save (PS)-Poll frame to an access point (AP); and receiving information in response to the PS-Poll frame from the access point (AP), wherein information indicating an access category (AC) of the PS-Poll frame is provided from the access point (AP) to the station (STA).

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/232,956, filed on Jan. 15, 2014, the entire disclosure ofwhich is hereby incorporated by reference for all purposes as if fullyset forth herein. U.S. patent application Ser. No. 14/232,956 is a U.S.National Stage Entry of PCT International Application No.PCT/KR2013/007939, filed on Sep. 3, 2013, and claims the benefit of U.S.Provisional Application No. 61/735,561, filed on Dec. 11, 2012; No.61/735,070, filed on Dec. 10, 2012; and No. 61/696,282, filed on Sep. 3,2012.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly to a method and apparatus for transmitting andreceiving power save-polling frame and response frame in a wireless LAN(WLAN) system.

BACKGROUND ART

Various wireless communication technologies systems have been developedwith rapid development of information communication technologies. WLANtechnology from among wireless communication technologies allowswireless Internet access at home or in enterprises or at a specificservice provision region using mobile terminals, such as a PersonalDigital Assistant (PDA), a laptop computer, a Portable Multimedia Player(PMP), etc. on the basis of Radio Frequency (RF) technology.

In order to obviate limited communication speed, one of the advantagesof WLAN, the recent technical standard has proposed an evolved systemcapable of increasing the speed and reliability of a network whilesimultaneously extending a coverage region of a wireless network. Forexample, Institute of Electrical and Electronics Engineers (IEEE)802.11n enables a data processing speed to support a maximum highthroughput (HT) of 540 Mbps. In addition, Multiple Input and MultipleOutput (MIMO) technology has recently been applied to both a transmitterand a receiver so as to minimize transmission errors as well as tooptimize a data transfer rate.

DISCLOSURE Technical Problem

Machine to Machine (M2M) communication technology has been discussed asnext generation communication technology. A technical standard forsupporting M2M communication in IEEE 802.11 WLAN has been developed asIEEE 802.11ah. M2M communication may consider a scenario capable ofcommunicating a small amount of data infrequently at low speed in anenvironment including a large number of devices.

Since stations (STAs) for use in a WLAN system can competitively accessa wireless medium, access priority or an access category may beestablished to provide a predetermined service quality. In order toprevent collision of a polling message (for example, PS-Poll frame) sentto an access point (AP) in a power saving mode of the legacy WLANsystem, a low-priority access category is used.

Differently from the PS-Poll frame transmission operation of the legacySTA, if STAs (for example, non-TIM STAs) capable of transmitting thePS-Poll frame without confirming a traffic indication map (TIM) areintroduced, applying the legacy access category without change mayencounter the problem of increasing STA power consumption.

The present invention provides a method for establishing a separateaccess category for transmission of the PS-Poll frame of the STA,preventing increase in power consumption of the STA, and at the sametime increasing the efficiency of channel access through the PS-Pollframe.

It is to be understood that technical objects to be achieved by thepresent invention are not limited to the aforementioned technicalobjects and other technical objects which are not mentioned herein willbe apparent from the following description to one of ordinary skill inthe art to which the present invention pertains.

Technical Solution

The object of the present invention can be achieved by providing amethod for transmitting a frame by a station (STA) in a wireless LAN(WLAN) system including: awakening at a predetermined time, andtransmitting a Power Save (PS)-Poll frame to an access point (AP); andreceiving information in response to the PS-Poll frame from the accesspoint (AP), wherein information indicating an access category (AC) ofthe PS-Poll frame is provided from the access point (AP) to the station(STA).

In accordance with another aspect of the present invention, a method forsupporting frame transmission by an access point (AP) in a wireless LAN(WLAN) system includes: receiving, by an access point (AP), a Power Save(PS)-Poll frame from the station (STA) awoken at a predetermined time;and transmitting information in response to the PS-Poll frame from theaccess point (AP) to the station (STA), wherein information indicatingan access category (AC) of the PS-Poll frame is provided from the accesspoint (AP) to the station (STA).

In accordance with another aspect of the present invention, a station(STA) device for transmitting a frame in a wireless LAN (WLAN) systemincludes: a transceiver; and a processor, wherein the processor isconfigured to awake at a predetermined time, transmit a Power Save(PS)-Poll frame to an access point (AP) using the transceiver, andreceive information in response to the PS-Poll frame from the accesspoint (AP) using the transceiver, wherein information indicating anaccess category (AC) of the PS-Poll frame is provided from the accesspoint (AP) to the station (STA).

In accordance with another aspect of the present invention, an accesspoint (AP) device for supporting frame transmission of a station (STA)in a wireless LAN (WLAN) system includes: a transceiver; and aprocessor, and the processor is configured to receive a Power Save(PS)-Poll frame from the station (STA) awoken at a predetermined timeusing the transceiver, and transmit information in response to thePS-Poll frame to the station (STA) using the transceiver, whereininformation indicating an access category (AC) of the PS-Poll frame isprovided from the access point (AP) to the station (STA).

The following description may be commonly applied to the embodiments ofthe present invention.

The information indicating the access category (AC) may be a PS-Poll ACfield.

The PS-Poll AC field may be used when the access point (AP) informs theSTA of an access category (AC) for the PS-Poll transmission.

The PS-Poll AC field may be contained in a beacon frame.

The PS-Poll AC field may be 2 bits long.

The PS-Poll frame may be transmitted using an access category (AC) basedon a specific value set by the PS-Poll AC field.

If the information indicating the access category (AC) of the PS-Pollframe is not provided to the STA, the PS-Poll frame may be transmittedusing an access category (AC) set as a default value.

The PS-Poll frame may be transmitted using an access category AC_VO(Access Category_Voice).

The predetermined time may be allocated by the access point (AP).

The station (STA) may be a non-TIM STA.

Transmission of the station (STA) may be allowed only within aRestricted Access Window (RAW) period allocated by the access point(AP).

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

As is apparent from the above description, exemplary embodiments of thepresent invention may allow a device operating in a WLAN system tocorrectly perform/support efficient sub-channel selective access.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present invention are not limited to whathas been particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 exemplarily shows an IEEE 802.11 system according to oneembodiment of the present invention.

FIG. 2 exemplarily shows an IEEE 802.11 system according to anotherembodiment of the present invention.

FIG. 3 exemplarily shows an IEEE 802.11 system according to stillanother embodiment of the present invention.

FIG. 4 is a conceptual diagram illustrating a WLAN system.

FIG. 5 is a flowchart illustrating a link setup process for use in theWLAN system.

FIG. 6 is a conceptual diagram illustrating a backoff process.

FIG. 7 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 8 is a conceptual diagram illustrating RTS (Request To Send) andCTS (Clear To Send).

FIG. 9 is a conceptual diagram illustrating a power managementoperation.

FIGS. 10 to 12 are conceptual diagrams illustrating operations of an STAhaving received a TIM.

FIG. 13 is a conceptual diagram illustrating a group based AID.

FIG. 14 is a conceptual diagram illustrating a frame structure for usein IEEE 802.11.

FIG. 15 is a conceptual diagram illustrating a Restricted Access Window(RAW) structure according to one example of the present invention.

FIG. 16 is a structural diagram illustrating an Information Element (IE)used for establishing the PS-Poll AC according to the present invention.

FIG. 17 is a conceptual diagram illustrating a PS-Poll response methodaccording to one example of the present invention.

FIG. 18 is a conceptual diagram illustrating a PS-Poll response methodaccording to another example of the present invention.

FIG. 19 is a flowchart illustrating the PS-Poll process according to oneexample of the present invention.

FIG. 20 is a block diagram illustrating a radio frequency (RF) deviceaccording to one embodiment of the present invention.

BEST MODE

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the present invention.The following detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details.

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. In addition, some constituent componentsand/or characteristics may be combined to implement the embodiments ofthe present invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3^(rd) Generation Partnership Project (3GPP) system, a3GPP Long Term Evolution (LTE) system, an LTE-Advanced (LTE-A) system,and a 3GPP2 system. In particular, steps or parts, which are notdescribed to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. All terminology used herein may be supported by atleast one of the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. CDMA may be embodied through wireless(or radio) technology such as UTRA (Universal Terrestrial Radio Access)or CDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as GSM (Global System for Mobile communication)/GPRS (GeneralPacket Radio Service)/EDGE (Enhanced Data Rates for GSM Evolution).OFDMA may be embodied through wireless (or radio) technology such asInstitute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi),IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). Forclarity, the following description focuses on IEEE 802.11 systems.However, technical features of the present invention are not limitedthereto.

WLAN System Structure

FIG. 1 exemplarily shows an IEEE 802.11 system according to oneembodiment of the present invention.

The structure of the IEEE 802.11 system may include a plurality ofcomponents. A WLAN which supports transparent STA mobility for a higherlayer may be provided by mutual operations of the components. A BasicService Set (BSS) may correspond to a basic constituent block in an IEEE802.11 LAN. In FIG. 1, two BSSs (BSS1 and BSS2) are shown and two STAsare included in each of the BSSs (i.e. STA1 and STA2 are included inBSS1 and STA3 and STA4 are included in BSS2). An ellipse indicating theBSS in FIG. 1 may be understood as a coverage area in which STAsincluded in the corresponding BSS maintain communication. This area maybe referred to as a Basic Service Area (BSA). If an STA moves out of theBSA, the STA cannot directly communicate with the other STAs in thecorresponding BSA.

In the IEEE 802.11 LAN, the most basic type of BSS is an Independent BSS(IBSS). For example, the IBSS may have a minimum form consisting of onlytwo STAs. The BSS (BSS1 or BSS2) of FIG. 1, which is the simplest formand in which other components are omitted, may correspond to a typicalexample of the IBSS. Such configuration is possible when STAs candirectly communicate with each other. Such a type of LAN is notprescheduled and may be configured when the LAN is necessary. This maybe referred to as an ad-hoc network.

Memberships of an STA in the BSS may be dynamically changed when the STAis switched on or off or the STA enters or leaves the BSS region. TheSTA may use a synchronization process to join the BSS. To access allservices of a BSS infrastructure, the STA should be associated with theBSS. Such association may be dynamically configured and may include useof a Distribution System Service (DSS).

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 2,components such as a Distribution System (DS), a Distribution SystemMedium (DSM), and an Access Point (AP) are added to the structure ofFIG. 1.

A direct STA-to-STA distance in a LAN may be restricted by PHYperformance. In some cases, such restriction of the distance may besufficient for communication. However, in other cases, communicationbetween STAs over a long distance may be necessary. The DS may beconfigured to support extended coverage.

The DS refers to a structure in which BSSs are connected to each other.Specifically, a BSS may be configured as a component of an extended formof a network consisting of a plurality of BSSs, instead of independentconfiguration as shown in FIG. 1.

The DS is a logical concept and may be specified by the characteristicof the DSM. In relation to this, a Wireless Medium (WM) and the DSM arelogically distinguished in IEEE 802.11. Respective logical media areused for different purposes and are used by different components. Indefinition of IEEE 802.11, such media are not restricted to the same ordifferent media. The flexibility of the IEEE 802.11 LAN architecture (DSarchitecture or other network architectures) can be explained in that aplurality of media is logically different. That is, the IEEE 802.11 LANarchitecture can be variously implemented and may be independentlyspecified by a physical characteristic of each implementation.

The DS may support mobile devices by providing seamless integration ofmultiple BSSs and providing logical services necessary for handling anaddress to a destination.

The AP refers to an entity that enables associated STAs to access the DSthrough a WM and that has STA functionality. Data may move between theBSS and the DS through the AP. For example, STA2 and STA3 shown in FIG.2 have STA functionality and provide a function of causing associatedSTAs (STA1 and STA4) to access the DS. Moreover, since all APscorrespond basically to STAs, all APs are addressable entities. Anaddress used by an AP for communication on the WM need not always beidentical to an address used by the AP for communication on the DSM.

Data transmitted from one of STAs associated with the AP to an STAaddress of the AP may always be received by an uncontrolled port and maybe processed by an IEEE 802.1X port access entity. If the controlledport is authenticated, transmission data (or frame) may be transmittedto the DS.

FIG. 3 is a diagram showing still another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In additionto the structure of FIG. 2, FIG. 3 conceptually shows an ExtendedService Set (ESS) for providing wide coverage.

A wireless network having arbitrary size and complexity may be comprisedof a DS and BSSs. In the IEEE 802.11 system, such a type of network isreferred to an ESS network. The ESS may correspond to a set of BSSsconnected to one DS. However, the ESS does not include the DS. The ESSnetwork is characterized in that the ESS network appears as an IBSSnetwork in a Logical Link Control (LLC) layer. STAs included in the ESSmay communicate with each other and mobile STAs are movabletransparently in LLC from one BSS to another BSS (within the same ESS).

In IEEE 802.11, relative physical locations of the BSSs in FIG. 3 arenot assumed and the following forms are all possible. BSSs may partiallyoverlap and this form is generally used to provide continuous coverage.BSSs may not be physically connected and the logical distances betweenBSSs have no limit. BSSs may be located at the same physical positionand this form may be used to provide redundancy. One or more IBSSs orESS networks may be physically located in the same space as one or moreESS networks. This may correspond to an ESS network form in the case inwhich an ad-hoc network operates in a location in which an ESS networkis present, the case in which IEEE 802.11 networks of differentorganizations physically overlap, or the case in which two or moredifferent access and security policies are necessary in the samelocation.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system. InFIG. 4, an example of an infrastructure BSS including a DS is shown.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In the WLANsystem, an STA is a device operating according to MAC/PHY regulation ofIEEE 802.11. STAs include AP STAs and non-AP STAs. The non-AP STAscorrespond to devices, such as laptop computers or mobile phones,handled directly by users. In FIG. 4, STA1, STA3, and STA4 correspond tothe non-AP STAs and STA2 and STA5 correspond to AP STAs.

In the following description, the non-AP STA may be referred to as aterminal, a Wireless Transmit/Receive Unit (WTRU), a User Equipment(UE), a Mobile Station (MS), a mobile terminal, or a Mobile SubscriberStation (MSS). The AP is a concept corresponding to a Base Station (BS),a Node-B, an evolved Node-B (e-NB), a Base Transceiver System (BTS), ora femto BS in other wireless communication fields.

Link Setup Process

FIG. 5 is a flowchart explaining a general link setup process accordingto an exemplary embodiment of the present invention.

In order to allow an STA to establish link setup on the network as wellas to transmit/receive data over the network, the STA must perform suchlink setup through processes of network discovery, authentication, andassociation, and must establish association and perform securityauthentication. The link setup process may also be referred to as asession initiation process or a session setup process. In addition, anassociation step is a generic term for discovery, authentication,association, and security setup steps of the link setup process.

Link setup process is described referring to FIG. 5.

In step S510, STA may perform the network discovery action. The networkdiscovery action may include the STA scanning action. That is, STA mustsearch for an available network so as to access the network. The STAmust identify a compatible network before participating in a wirelessnetwork. Here, the process for identifying the network contained in aspecific region is referred to as a scanning process.

The scanning scheme is classified into active scanning and passivescanning.

FIG. 5 is a flowchart illustrating a network discovery action includingan active scanning process. In the case of the active scanning, an STAconfigured to perform scanning transmits a probe request frame and waitsfor a response to the probe request frame, such that the STA can movebetween channels and at the same time can determine which AP (AccessPoint) is present in a peripheral region. A responder transmits a proberesponse frame, acting as a response to the probe request frame, to theSTA having transmitted the probe request frame. In this case, theresponder may be an STA that has finally transmitted a beacon frame in aBSS of the scanned channel. In BSS, since the AP transmits the beaconframe, the AP operates as a responder. In IBSS, since STAs of the IBSSsequentially transmit the beacon frame, the responder is not constant.For example, the STA, that has transmitted the probe request frame atChannel #1 and has received the probe response frame at Channel #1,stores BSS-associated information contained in the received proberesponse frame, and moves to the next channel (for example, Channel #2),such that the STA may perform scanning using the same method (i.e.,probe request/response transmission/reception at Channel #2).

Although not shown in FIG. 5, the scanning action may also be carriedout using passive scanning. An STA configured to perform scanning in thepassive scanning mode waits for a beacon frame while simultaneouslymoving from one channel to another channel. The beacon frame is one ofmanagement frames in IEEE 802.11, indicates the presence of a wirelessnetwork, enables the STA performing scanning to search for the wirelessnetwork, and is periodically transmitted in a manner that the STA canparticipate in the wireless network. In BSS, the AP is configured toperiodically transmit the beacon frame. In IBSS, STAs of the IBSS areconfigured to sequentially transmit the beacon frame. If each STA forscanning receives the beacon frame, the STA stores BSS informationcontained in the beacon frame, and moves to another channel and recordsbeacon frame information at each channel. The STA having received thebeacon frame stores BSS-associated information contained in the receivedbeacon frame, moves to the next channel, and thus performs scanningusing the same method.

In comparison between the active scanning and the passive scanning, theactive scanning is more advantageous than the passive scanning in termsof delay and power consumption.

After the STA discovers the network, the STA may perform theauthentication process in step S520. The authentication process may bereferred to as a first authentication process in such a manner that theauthentication process can be clearly distinguished from the securitysetup process of step S540.

The authentication process may include transmitting an authenticationrequest frame to an AP by the STA, and transmitting an authenticationresponse frame to the STA by the AP in response to the authenticationrequest frame. The authentication frame used for authenticationrequest/response may correspond to a management frame.

The authentication frame may include an authentication algorithm number,an authentication transaction sequence number, a state code, a challengetext, a Robust Security Network (RSN), a Finite Cyclic Group (FCG), etc.The above-mentioned information contained in the authentication framemay correspond to some parts of information capable of being containedin the authentication request/response frame, may be replaced with otherinformation, or may include additional information.

The STA may transmit the authentication request frame to the AP. The APmay decide whether to authenticate the corresponding STA on the basis ofinformation contained in the received authentication request frame. TheAP may provide the authentication result to the STA through theauthentication response frame.

After the STA has been successfully authenticated, the associationprocess may be carried out in step S530. The association process mayinvolve transmitting an association request frame to the AP by the STA,and transmitting an association response frame to the STA by the AP inresponse to the association request frame.

For example, the association request frame may include informationassociated with various capabilities, a beacon listen interval, aService Set Identifier (SSID), supported rates, supported channels, RSN,mobility domain, supported operating classes, a TIM (Traffic IndicationMap) broadcast request, interworking service capability, etc.

For example, the association response frame may include informationassociated with various capabilities, a state code, an Association ID(AID), supported rates, an Enhanced Distributed Channel Access (EDCA)parameter set, a Received Channel Power Indicator (RCPI), a ReceivedSignal to Noise Indicator (RSNI), mobility domain, a timeout interval(association comeback time), an overlapping BSS scan parameter, a TIMbroadcast response, a QoS map, etc.

The above-mentioned information may correspond to some parts ofinformation capable of being contained in the associationrequest/response frame, may be replaced with other information, or mayinclude additional information.

After the STA has been successfully associated with the network, asecurity setup process may be carried out in step S540. The securitysetup process of Step S540 may be referred to as an authenticationprocess based on Robust Security Network Association (RSNA)request/response. The authentication process of step S520 may bereferred to as a first authentication process, and the security setupprocess of Step S540 may also be simply referred to as an authenticationprocess.

For example, the security setup process of Step S540 may include aprivate key setup process through 4-way handshaking based on an(Extensible Authentication Protocol over LAN (EAPOL) frame. In addition,the security setup process may also be carried out according to othersecurity schemes not defined in IEEE 802.11 standards.

WLAN Evolution

In order to obviate limitations in WLAN communication speed, IEEE802.11n has recently been established as a communication standard. IEEE802.11n aims to increase network speed and reliability as well as toextend a coverage region of the wireless network. In more detail, IEEE802.11n supports a High Throughput (HT) of a maximum of 540 Mbps, and isbased on MIMO technology in which multiple antennas are mounted to eachof a transmitter and a receiver.

In order to efficiently utilize a radio frequency (RF) channel, the nextgeneration WLAN system supports MU-MIMO (Multi User Multiple InputMultiple Output) transmission in which a plurality of STAs cansimultaneously access a channel. In accordance with the MU-MIMOtransmission scheme, the AP may simultaneously transmit packets to atleast one MIMO-paired STA.

In addition, a technology for supporting WLAN system operations inwhitespace has recently been discussed. For example, a technology forintroducing the WLAN system in whitespace (TV WS) such as an idlefrequency band (for example, 54˜698 MHz band) left because of thetransition to digital TV has been discussed under the IEEE 802.11afstandard. However, the above-mentioned information is disclosed forillustrative purposes only, and the whitespace may be a licensed bandcapable of being primarily used only by a licensed user. The licenseduser may be a user who has authority to use the licensed band, and mayalso be referred to as a licensed device, a primary user, an incumbentuser, or the like.

For example, an AP and/or STA operating in the whitespace (WS) mustprovide a function for protecting the licensed user. For example,assuming that the licensed user such as a microphone has already used aspecific WS channel acting as a divided frequency band on regulation ina manner that a specific bandwidth is occupied from the WS band, the APand/or STA cannot use the frequency band corresponding to thecorresponding WS channel so as to protect the licensed user. Inaddition, the AP and/or STA must stop using the corresponding frequencyband under the condition that the licensed user uses a frequency bandused for transmission and/or reception of a current frame.

Therefore, the AP and/or STA must determine whether to use a specificfrequency band of the WS band. In other words, the AP and/or STA mustdetermine the presence or absence of an incumbent user or a licenseduser in the frequency band. The scheme for determining the presence orabsence of the incumbent user in a specific frequency band is referredto as a spectrum sensing scheme. An energy detection scheme, a signaturedetection scheme and the like may be used as the spectrum sensingmechanism. The AP and/or STA may determine that the frequency band isbeing used by an incumbent user if the intensity of a received signalexceeds a predetermined value, or when a DTV preamble is detected.

M2M (Machine to Machine) communication technology has been discussed asnext generation communication technology. Technical standard forsupporting M2M communication has been developed as IEEE 802.11ah in theIEEE 802.11 WLAN system. M2M communication refers to a communicationscheme including one or more machines, or may also be referred to asMachine Type Communication (MTC) or Machine To Machine (M2M)communication. In this case, the machine may be an entity that does notrequire direct handling and intervention of a user. For example, notonly a meter or vending machine including a RF module, but also a userequipment (UE) (such as a smartphone) capable of performingcommunication by automatically accessing the network without userintervention/handling may be an example of such machines. M2Mcommunication may include Device-to-Device (D2 D) communication andcommunication between a device and an application server, etc. Asexemplary communication between the device and the application server,communication between a vending machine and an application server,communication between the Point of Sale (POS) device and the applicationserver, and communication between an electric meter, a gas meter or awater meter and the application server. M2M-based communicationapplications may include security, transportation, healthcare, etc. Inthe case of considering the above-mentioned application examples, M2Mcommunication has to support the method for sometimestransmitting/receiving a small amount of data at low speed under anenvironment including a large number of devices.

In more detail, M2M communication must support a large number of STAs.Although the current WLAN system assumes that one AP is associated witha maximum of 2007 STAs, various methods for supporting other cases inwhich many more STAs (e.g., about 6000 STAs) are associated with one APhave recently been discussed in M2M communication. In addition, it isexpected that many applications for supporting/requesting a low transferrate are present in M2M communication. In order to smoothly support manySTAs, the WLAN system may recognize the presence or absence of data tobe transmitted to the STA on the basis of a TIM (Traffic Indicationmap), and various methods for reducing the bitmap size of the TIM haverecently been discussed. In addition, it is expected that much trafficdata having a very long transmission/reception interval is present inM2M communication. For example, in M2M communication, a very smallamount of data (e.g., electric/gas/water metering) needs to betransmitted at long intervals (for example, every month). Therefore,although the number of STAs associated with one AP increases in the WLANsystem, many developers and companies are conducting intensive researchinto an WLAN system which can efficiently support the case in whichthere are a very small number of STAs, each of which has a data frame tobe received from the AP during one beacon period.

As described above, WLAN technology is rapidly developing, and not onlythe above-mentioned exemplary technologies but also other technologiessuch as a direct link setup, improvement of media streaming throughput,high-speed and/or support of large-scale initial session setup, andsupport of extended bandwidth and operation frequency, are beingintensively developed.

Medium Access Mechanism

In the IEEE 802.11—based WLAN system, a basic access mechanism of MAC(Medium Access Control) is a Carrier Sense Multiple Access withCollision Avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism isreferred to as a Distributed Coordination Function (DCF) of IEEE 802.11MAC, and basically includes a “Listen Before Talk” access mechanism. Inaccordance with the above-mentioned access mechanism, the AP and/or STAmay perform Clear Channel Assessment (CCA) for sensing an RF channel ormedium during a predetermined time interval [for example, DCFInter-Frame Space (DIFS)], prior to data transmission. If it isdetermined that the medium is in the idle state, frame transmissionthrough the corresponding medium begins. On the other hand, if it isdetermined that the medium is in the occupied state, the correspondingAP and/or STA does not start its own transmission, establishes a delaytime (for example, a random backoff period) for medium access, andattempts to start frame transmission after waiting for a predeterminedtime. Through application of a random backoff period, it is expectedthat multiple STAs will attempt to start frame transmission afterwaiting for different times, resulting in minimum collision.

In addition, IEEE 802.11 MAC protocol provides a Hybrid CoordinationFunction (HCF). HCF is based on DCF and Point Coordination Function(PCF). PCF refers to the polling-based synchronous access scheme inwhich periodic polling is executed in a manner that all reception (Rx)APs and/or STAs can receive the data frame. In addition, HCF includesEnhanced Distributed Channel Access (EDCA) and HCF Controlled ChannelAccess (HCCA). EDCA is achieved when the access scheme provided from aprovider to a plurality of users is contention-based. HCCA is achievedby the contention-free-based channel access scheme based on the pollingmechanism. In addition, HCF includes a medium access mechanism forimproving Quality of Service (QoS) of WLAN, and may transmit QoS data inboth a Contention Period (CP) and a Contention Free Period (CFP).

FIG. 6 is a conceptual diagram illustrating a backoff process.

Operations based on a random backoff period will hereinafter bedescribed with reference to FIG. 6. If the occupy- or busy-state mediumis shifted to an idle state, several STAs may attempt to transmit data(or frame). As a method for implementing a minimum number of collisions,each STA selects a random backoff count, waits for a slot timecorresponding to the selected backoff count, and then attempts to startdata transmission. The random backoff count has a value of a packetnumber, and may be set to one of 0 to CW values. In this case, CW refersto a Contention Window parameter value. Although an initial value of theCW parameter is denoted by CWmin, the initial value may be doubled incase of a transmission failure (for example, in the case in which ACK ofthe transmission frame is not received). If the CW parameter value isdenoted by CWmax, CWmax is maintained until data transmission issuccessful, and at the same time it is possible to attempt to start datatransmission. If data transmission was successful, the CW parametervalue is reset to CWmin. Preferably, CW, CWmin, and CWmax are set to2^(n)−1 (where n=0, 1, 2, . . . ).

If the random backoff process starts operation, the STA continuouslymonitors the medium while counting down the backoff slot in response tothe decided backoff count value. If the medium is monitored as theoccupied state, the countdown stops and waits for a predetermined time.If the medium is in the idle state, the remaining countdown restarts.

As shown in the example of FIG. 6, if a packet to be transmitted to MACof STA3 arrives at the STA3, the STA3 determines whether the medium isin the idle state during the DIFS, and may directly start frametransmission. In the meantime, the remaining STAs monitor whether themedium is in the busy state, and wait for a predetermined time. Duringthe predetermined time, data to be transmitted may occur in each ofSTA1, STA2, and STA5. If the medium is in the idle state, each STA waitsfor the DIFS time and then performs countdown of the backoff slot inresponse to a random backoff count value selected by each STA. Theexample of FIG. 6 shows that STA2 selects the lowest backoff count valueand STA1 selects the highest backoff count value. That is, after STA2finishes backoff counting, the residual backoff time of STA5 at a frametransmission start time is shorter than the residual backoff time ofSTA1. Each of STA1 and STA5 temporarily stops countdown while STA2occupies the medium, and waits for a predetermined time. If occupying ofthe STA2 is finished and the medium re-enters the idle state, each ofSTA1 and STA5 waits for a predetermined time DIFS, and restarts backoffcounting. That is, after the remaining backoff slot as long as theresidual backoff time is counted down, frame transmission may startoperation. Since the residual backoff time of STA5 is shorter than thatof STA1, STA5 starts frame transmission. Meanwhile, data to betransmitted may occur in STA4 while STA2 occupies the medium. In thiscase, if the medium is in the idle state, STA4 waits for the DIFS time,performs countdown in response to the random backoff count valueselected by the STA4, and then starts frame transmission. FIG. 6exemplarily shows the case in which the residual backoff time of STA5 isidentical to the random backoff count value of STA4 by chance. In thiscase, an unexpected collision may occur between STA4 and STA5. If thecollision occurs between STA4 and STA5, each of STA4 and STA5 does notreceive ACK, resulting in the occurrence of a failure in datatransmission. In this case, each of STA4 and STA5 increases the CW valuetwo times, and STA4 or STA5 may select a random backoff count value andthen perform countdown. Meanwhile, STA1 waits for a predetermined timewhile the medium is in the occupied state due to transmission of STA4and STA5. In this case, if the medium is in the idle state, STA1 waitsfor the DIFS time, and then starts frame transmission after lapse of theresidual backoff time.

STA Sensing Operation

As described above, the CSMA/CA mechanism includes not only a physicalcarrier sensing mechanism in which the AP and/or STA can directly sensethe medium, but also a virtual carrier sensing mechanism. The virtualcarrier sensing mechanism can solve some problems (such as a hidden nodeproblem) encountered in the medium access. For the virtual carriersensing, MAC of the WLAN system can utilize a Network Allocation Vector(NAV). In more detail, by means of the NAV value, the AP and/or STA,each of which currently uses the medium or has authority to use themedium, may inform another AP and/or another STA for the remaining timein which the medium is available. Accordingly, the NAV value maycorrespond to a reserved time in which the medium will be used by the APand/or STA configured to transmit the corresponding frame. An STA havingreceived the NAV value is prohibited for medium access during thecorresponding reserved time. For example, NAV may be set according tothe value of a ‘duration’ field of the MAC header of the frame.

The robust collision detect mechanism has been proposed to reduce theprobability of such collision, and as such a detailed descriptionthereof will hereinafter be described with reference to FIGS. 7 and 8.Although an actual carrier sensing range is different from atransmission range, it is assumed that the actual carrier sensing rangeis identical to the transmission range for convenience of descriptionand better understanding of the present invention.

FIG. 7 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 7(a) exemplarily shows the hidden node. In FIG. 7(a), STA Acommunicates with STA B, and STA C has information to be transmitted. InFIG. 7(a), STA C may determine that the medium is in the idle state whenperforming carrier sensing before transmitting data to STA B, under thecondition that STA A transmits information to STA B. Since transmissionof STA A (i.e., occupied medium) may not be detected at the location ofSTA C, it is determined that the medium is in the idle state. In thiscase, STA B simultaneously receives information of STA A and informationof STA C, resulting in the occurrence of collision. Here, STA A may beconsidered as a hidden node of STA C.

FIG. 7(b) exemplarily shows an exposed node. In FIG. 7(b), under thecondition that STA B transmits data to STA A, STA C has information tobe transmitted to STA D. If STA C performs carrier sensing, it isdetermined that the medium is occupied due to transmission of STA B.Therefore, although STA C has information to be transmitted to STA D,the medium-occupied state is sensed, such that the STA C must wait for apredetermined time (i.e., standby mode) until the medium is in the idlestate. However, since STA A is actually located out of the transmissionrange of STA C, transmission from STA C may not collide withtransmission from STA B from the viewpoint of STA A, such that STA Cunnecessarily enters the standby mode until STA B stops transmission.Here, STA C is referred to as an exposed node of STA B.

FIG. 8 is a conceptual diagram illustrating RTS (Request To Send) andCTS (Clear To Send).

In order to efficiently utilize the collision avoidance mechanism underthe above-mentioned situation of FIG. 7, it is possible to use a shortsignaling packet such as RTS (request to send) and CTS (clear to send).RTS/CTS between two STAs may be overheared by peripheral STA(s), suchthat the peripheral STA(s) may consider whether information iscommunicated between the two STAs. For example, if STA to be used fordata transmission transmits the RTS frame to the STA having receiveddata, the STA having received data transmits the CTS frame to peripheralSTAs, and may inform the peripheral STAs that the STA is going toreceive data.

FIG. 8(a) exemplarily shows the method for solving problems of thehidden node. In FIG. 8(a), it is assumed that each of STA A and STA C isready to transmit data to STA B. If STA A transmits RTS to STA B, STA Btransmits CTS to each of STA A and STA C located in the vicinity of theSTA B. As a result, STA C must wait for a predetermined time until STA Aand STA B stop data transmission, such that collision is prevented fromoccurring.

FIG. 8(b) exemplarily shows the method for solving problems of theexposed node. STA C performs overhearing of RTS/CTS transmission betweenSTA A and STA B, such that STA C may determine no collision although ittransmits data to another STA (for example, STA D). That is, STA Btransmits an RTS to all peripheral STAs, and only STA A having data tobe actually transmitted can transmit a CTS. STA C receives only the RTSand does not receive the CTS of STA A, such that it can be recognizedthat STA A is located outside of the carrier sensing range of STA C.

Power Management

As described above, the WLAN system has to perform channel sensingbefore STA performs data transmission/reception. The operation of alwayssensing the channel causes persistent power consumption of the STA.There is not much difference in power consumption between the reception(Rx) state and the transmission (Tx) state. Continuous maintenance ofthe Rx state may cause large load to a power-limited STA (i.e., STAoperated by a battery). Therefore, if STA maintains the Rx standby modeso as to persistently sense the channel, power is inefficiently consumedwithout special advantages in terms of WLAN throughput. In order tosolve the above-mentioned problem, the WLAN system supports a powermanagement (PM) mode of the STA.

The PM mode of the STA is classified into an active mode and a PowerSave (PS) mode. The STA is basically operated in the active mode. TheSTA operating in the active mode maintains an awake state. If the STA isin the awake state, the STA may normally operate such that it canperform frame transmission/reception, channel scanning, or the like. Onthe other hand, STA operating in the PS mode is configured to switchfrom the doze state to the awake state or vice versa. STA operating inthe sleep state is operated with minimum power, and the STA does notperform frame transmission/reception and channel scanning.

The amount of power consumption is reduced in proportion to a specifictime in which the STA stays in the sleep state, such that the STAoperation time is increased in response to the reduced powerconsumption. However, it is impossible to transmit or receive the framein the sleep state, such that the STA cannot mandatorily operate for along period of time. If there is a frame to be transmitted to the AP,the STA operating in the sleep state is switched to the awake state,such that it can transmit/receive the frame in the awake state. On theother hand, if the AP has a frame to be transmitted to the STA, thesleep-state STA is unable to receive the frame and cannot recognize thepresence of a frame to be received. Accordingly, STA may need to switchto the awake state according to a specific period in order to recognizethe presence or absence of a frame to be transmitted to the STA (or inorder to receive a signal indicating the presence of the frame on theassumption that the presence of the frame to be transmitted to the STAis decided).

FIG. 9 is a conceptual diagram illustrating a power management (PM)operation.

Referring to FIG. 9, AP 210 transmits a beacon frame to STAs present inthe BSS at intervals of a predetermined time period in steps (S211,S212, S213, S214, S215, S216). The beacon frame includes a TIMinformation element. The TIM information element includes bufferedtraffic regarding STAs associated with the AP 210, and includes specificinformation indicating that a frame is to be transmitted. The TIMinformation element includes a TIM for indicating a unicast frame and aDelivery Traffic Indication Map (DTIM) for indicating a multicast orbroadcast frame.

AP 210 may transmit a DTIM once whenever the beacon frame is transmittedthree times. Each of STA1 220 and STA2 222 is operated in the PS mode.Each of STA1 220 and STA2 222 is switched from the sleep state to theawake state every wakeup interval, such that STA1 220 and STA2 222 maybe configured to receive the TIM information element transmitted by theAP 210. Each STA may calculate a switching start time at which each STAmay start switching to the awake state on the basis of its own localclock. In FIG. 9, it is assumed that a clock of the STA is identical toa clock of the AP.

For example, the predetermined wakeup interval may be configured in sucha manner that STA1 220 can switch to the awake state to receive the TIMelement every beacon interval. Accordingly, STA1 220 may switch to theawake state in step S221 when AP 210 first transmits the beacon frame instep S211. STA1 220 receives the beacon frame, and obtains the TIMinformation element. If the obtained TIM element indicates the presenceof a frame to be transmitted to STA1 220, STA1 220 may transmit a PowerSave-Poll (PS-Poll) frame, which requests the AP 210 to transmit theframe, to the AP 210 in step S221 a. The AP 210 may transmit the frameto STA 1 220 in response to the PS-Poll frame in step S231. STA1 220having received the frame is re-switched to the sleep state, andoperates in the sleep state.

When AP 210 secondly transmits the beacon frame, a busy medium state inwhich the medium is accessed by another device is obtained, the AP 210may not transmit the beacon frame at an accurate beacon interval and maytransmit the beacon frame at a delayed time in step S212. In this case,although STA1 220 is switched to the awake state in response to thebeacon interval, it does not receive the delay-transmitted beacon frameso that it re-enters the sleep state in step S222.

When AP 210 thirdly transmits the beacon frame, the corresponding beaconframe may include a TIM element denoted by DTIM. However, since the busymedium state is given, AP 210 transmits the beacon frame at a delayedtime in step S213. STA1 220 is switched to the awake state in responseto the beacon interval, and may obtain a DTIM through the beacon frametransmitted by the AP 210. It is assumed that DTIM obtained by STA1 220does not have a frame to be transmitted to STA1 220 and there is a framefor another STA. In this case, STA1 220 confirms the absence of a frameto be received in the STA1 220, and re-enters the sleep state, such thatthe STA1 220 may operate in the sleep state. After the AP 210 transmitsthe beacon frame, the AP 210 transmits the frame to the correspondingSTA in step S232.

AP 210 fourthly transmits the beacon frame in step S214. However, it isimpossible for STA1 220 to obtain information regarding the presence ofbuffered traffic associated with the STA1 220 through double receptionof a TIM element, such that the STA1 220 may adjust the wakeup intervalfor receiving the TIM element. Alternatively, provided that signalinginformation for coordination of the wakeup interval value of STA1 220 iscontained in the beacon frame transmitted by AP 210, the wakeup intervalvalue of the STA1 220 may be adjusted. In this example, STA1 220, thathas been switched to receive a TIM element every beacon interval, may beswitched to another operation state in which STA1 220 can awake from thesleep state once every three beacon intervals. Therefore, when AP 210transmits a fourth beacon frame in step S214 and transmits a fifthbeacon frame in step S215, STA1 220 maintains the sleep state such thatit cannot obtain the corresponding TIM element.

When AP 210 sixthly transmits the beacon frame in step S216, STA1 220 isswitched to the awake state and operates in the awake state, such thatthe STA1 220 is unable to obtain the TIM element contained in the beaconframe in step S224. The TIM element is a DTIM indicating the presence ofa broadcast frame, such that STA1 220 does not transmit the PS-Pollframe to the AP 210 and may receive a broadcast frame transmitted by theAP 210 in step S234. In the meantime, the wakeup interval of STA2 230may be longer than a wakeup interval of STA1 220. Accordingly, STA2 230enters the awake state at a specific time S215 where the AP 210 fifthlytransmits the beacon frame, such that the STA2 230 may receive the TIMelement in step S241. STA2 230 recognizes the presence of a frame to betransmitted to the STA2 230 through the TIM element, and transmits thePS-Poll frame to the AP 210 so as to request frame transmission in stepS241 a. AP 210 may transmit the frame to STA2 230 in response to thePS-Poll frame in step S233.

In order to operate/manage the power save (PS) mode shown in FIG. 9, theTIM element may include either a TIM indicating the presence or absenceof a frame to be transmitted to the STA, or a DTIM indicating thepresence or absence of a broadcast/multicast frame. DTIM may beimplemented through field setting of the TIM element.

FIGS. 10 to 12 are conceptual diagrams illustrating detailed operationsof the STA having received a Traffic Indication Map (TIM).

Referring to FIG. 10, STA is switched from the sleep state to the awakestate so as to receive the beacon frame including a TIM from the AP. STAinterprets the received TIM element such that it can recognize thepresence or absence of buffered traffic to be transmitted to the STA.After STA contends with other STAs to access the medium for PS-Pollframe transmission, the STA may transmit the PS-Poll frame forrequesting data frame transmission to the AP. The AP having received thePS-Poll frame transmitted by the STA may transmit the frame to the STA.STA may receive a data frame and then transmit an ACK frame to the AP inresponse to the received data frame. Thereafter, the STA may re-enterthe sleep state.

As can be seen from FIG. 10, the AP may operate according to theimmediate response scheme, such that the AP receives the PS-Poll framefrom the STA and transmits the data frame after lapse of a predeterminedtime [for example, Short Inter-Frame Space (SIFS)]. In contrast, the APhaving received the PS-Poll frame does not prepare a data frame to betransmitted to the STA during the SIFS time, such that the AP mayoperate according to the deferred response scheme, and as such adetailed description thereof will hereinafter be described withreference to FIG. 11.

The STA operations of FIG. 11 in which the STA is switched from thesleep state to the awake state, receives a TIM from the AP, andtransmits the PS-Poll frame to the AP through contention are identicalto those of FIG. 10. If the AP having received the PS-Poll frame doesnot prepare a data frame during the SIFS time, the AP may transmit theACK frame to the STA instead of transmitting the data frame. If the dataframe is prepared after transmission of the ACK frame, the AP maytransmit the data frame to the STA after completion of such contending.STA may transmit the ACK frame indicating successful reception of a dataframe to the AP, and may be shifted to the sleep state.

FIG. 12 shows the exemplary case in which AP transmits DTIM. STAs may beswitched from the sleep state to the awake state so as to receive thebeacon frame including a DTIM element from the AP. STAs may recognizethat multicast/broadcast frame(s) will be transmitted through thereceived DTIM. After transmission of the beacon frame including theDTIM, AP may directly transmit data (i.e., multicast/broadcast frame)without transmitting/receiving the PS-Poll frame. While STAscontinuously maintains the awake state after reception of the beaconframe including the DTIM, the STAs may receive data, and then switch tothe sleep state after completion of data reception.

TIM Structure

In the operation and management method of the Power save (PS) mode basedon the TIM (or DTIM) protocol shown in FIGS. 9 to 12, STAs may determinethe presence or absence of a data frame to be transmitted for the STAsthrough STA identification information contained in the TIM element. STAidentification information may be specific information associated withan Association Identifier (AID) to be allocated when an STA isassociated with an AP.

AID is used as a unique ID of each STA within one BSS. For example, AIDfor use in the current WLAN system may be allocated to one of 1 to 2007.In the case of the current WLAN system, 14 bits for AID may be allocatedto a frame transmitted by AP and/or STA. Although the AID value may beassigned a maximum of 16383, the values of 2008˜16383 are set toreserved values.

The TIM element according to legacy definition is inappropriate forapplication of M2M application through which many STAs (for example, atleast 2007 STAs) are associated with one AP. If the conventional TIMstructure is extended without any change, the TIM bitmap sizeexcessively increases, such that it is impossible to support theextended TIM structure using the legacy frame format, and the extendedTIM structure is inappropriate for M2M communication in whichapplication of a low transfer rate is considered. In addition, it isexpected that there are a very small number of STAs each having an Rxdata frame during one beacon period. Therefore, according to exemplaryapplication of the above-mentioned M2M communication, it is expectedthat the TIM bitmap size is increased and most bits are set to zero (0),such that there is needed a technology capable of efficientlycompressing such bitmap.

In the legacy bitmap compression technology, successive values (each ofwhich is set to zero) of 0 are omitted from a head part of bitmap, andthe omitted result may be defined as an offset (or start point) value.However, although STAs each including the buffered frame is small innumber, if there is a high difference between AID values of respectiveSTAs, compression efficiency is not high. For example, assuming that theframe to be transmitted to only a first STA having an AID of 10 and asecond STA having an AID of 2000 is buffered, the length of a compressedbitmap is set to 1990, the remaining parts other than both edge partsare assigned zero (0). If STAs associated with one AP is small innumber, inefficiency of bitmap compression does not cause seriousproblems. However, if the number of STAs associated with one APincreases, such inefficiency may deteriorate overall system throughput.

In order to solve the above-mentioned problems, AIDs are divided into aplurality of groups such that data can be more efficiently transmittedusing the AIDs. A designated group ID (GID) is allocated to each group.AIDs allocated on the basis of such group will hereinafter be describedwith reference to FIG. 13.

FIG. 13(a) is a conceptual diagram illustrating a group-based AID. InFIG. 13(a), some bits located at the front part of the AID bitmap may beused to indicate a group ID (GID). For example, it is possible todesignate four GIDs using the first two bits of an AID bitmap. If atotal length of the AID bitmap is denoted by N bits, the first two bits(B1 and B2) may represent a GID of the corresponding AID.

FIG. 13(b) is a conceptual diagram illustrating a group-based AID. InFIG. 13(b), a GID may be allocated according to the position of AID. Inthis case, AIDs having the same GID may be represented by offset andlength values. For example, if GID 1 is denoted by Offset A and LengthB, this means that AIDs (A˜A+B−1) on bitmap are respectively set to GID1. For example, FIG. 13(b) assumes that AIDs (1˜N4) are divided intofour groups. In this case, AIDs contained in GID 1 are denoted by 1˜N1,and the AIDs contained in this group may be represented by Offset 1 andLength N1. AIDs contained in GID 2 may be represented by Offset (N1+1)and Length (N2−N1+1), AIDs contained in GID 3 may be represented byOffset (N2+1) and Length (N3−N2+1), and AIDs contained in GID 4 may berepresented by Offset (N3+1) and Length (N4−N3+1).

In case of using the aforementioned group-based AIDs, channel accessg isallowed in a different time interval according to individual GIDs, theproblem caused by the insufficient number of TIM elements compared witha large number of STAs can be solved and at the same time data can beefficiently transmitted/received. For example, during a specific timeinterval, channel access is allowed only for STA(s) corresponding to aspecific group, and channel access to the remaining STA(s) may berestricted. A predetermined time interval in which access to onlyspecific STA(s) is allowed may also be referred to as a RestrictedAccess Window (RAW).

Channel access based on GID will hereinafter be described with referenceto FIG. 13(c). If AIDs are divided into three groups, the channel accessmechanism according to the beacon interval is exemplarily shown in FIG.13(c). A first beacon interval (or a first RAW) is a specific intervalin which channel access to an STA corresponding to an AID contained inGID 1 is allowed, and channel access of STAs contained in other GIDs isdisallowed. For implementation of the above-mentioned structure, a TIMelement used only for AIDs corresponding to GID 1 is contained in afirst beacon frame. A TIM element used only for AIDs corresponding toGID 2 is contained in a second beacon frame. Accordingly, only channelaccess to an STA corresponding to the AID contained in GID 2 is allowedduring a second beacon interval (or a second RAW) during a second beaconinterval (or a second RAW). A TIM element used only for AIDs having GID3 is contained in a third beacon frame, such that channel access to anSTA corresponding to the AID contained in GID 3 is allowed using a thirdbeacon interval (or a third RAW). A TIM element used only for AIDs eachhaving GID 1 is contained in a fourth beacon frame, such that channelaccess to an STA corresponding to the AID contained in GID 1 is allowedusing a fourth beacon interval (or a fourth RAW). Thereafter, onlychannel access to an STA corresponding to a specific group indicated bythe TIM contained in the corresponding beacon frame may be allowed ineach of beacon intervals subsequent to the fifth beacon interval (or ineach of RAWs subsequent to the fifth RAW).

Although FIG. 13(c) exemplarily shows that the order of allowed GIDs isperiodical or cyclical according to the beacon interval, the scope orspirit of the present invention is not limited thereto. That is, onlyAID(s) contained in specific GID(s) may be contained in a TIM element,such that channel access to STA(s) corresponding to the specific AID(s)is allowed during a specific time interval (for example, a specificRAW), and channel access to the remaining STA(s) is disallowed.

The aforementioned group-based AID allocation scheme may also bereferred to as a hierarchical structure of a TIM. That is, a total AIDspace is divided into a plurality of blocks, and channel access toSTA(s) (i.e., STA(s) of a specific group) corresponding to a specificblock having any one of the remaining values other than ‘0’ may beallowed. Therefore, a large-sized TIM is divided into small-sizedblocks/groups, STA can easily maintain TIM information, andblocks/groups may be easily managed according to class, QoS or usage ofthe STA. Although FIG. 13 exemplarily shows a 2-level layer, ahierarchical TIM structure comprised of two or more levels may beconfigured. For example, a total AID space may be divided into aplurality of page groups, each page group may be divided into aplurality of blocks, and each block may be divided into a plurality ofsub-blocks. In this case, according to the extended version of FIG.13(a), first N1 bits of AID bitmap may represent a page ID (i.e., PID),the next N2 bits may represent a block ID, the next N3 bits mayrepresent a sub-block ID, and the remaining bits may represent theposition of STA bits contained in a sub-block.

In the examples of the present invention, various schemes for dividingSTAs (or AIDs allocated to respective STAs) into predeterminedhierarchical group units, and managing the divided result may be appliedto the embodiments, however, the group-based AID allocation scheme isnot limited to the above examples.

Frame Structure

FIG. 14 is a diagram for explaining an exemplary frame format used in802.11 system.

A Physical Layer Convergence Protocol (PLCP) Packet Data Unit (PPDU)frame format may include a Short Training Field (STF), a Long TrainingField (LTF), a signal (SIG) field, and a data field. The most basic (forexample, non-HT) PPDU frame format may be comprised of a Legacy-STF(L-STF) field, a Legacy-LTF (L-LTF) field, an SIG field, and a datafield. In addition, the most basic PPDU frame format may further includeadditional fields (i.e., STF, LTF, and SIG fields) between the SIG fieldand the data field according to the PPDU frame format types (forexample, HT-mixed format PPDU, HT-greenfield format PPDU, a VHT PPDU,and the like).

STF is a signal for signal detection, Automatic Gain Control (AGC),diversity selection, precise time synchronization, etc. LTF is a signalfor channel estimation, frequency error estimation, etc. The sum of STFand LTF may be referred to as a PCLP preamble. The PLCP preamble may bereferred to as a signal for synchronization and channel estimation of anOFDM physical layer.

The SIG field may include a RATE field, a LENGTH field, etc. The RATEfield may include information regarding data modulation and coding rate.The LENGTH field may include information regarding the length of data.Furthermore, the SIG field may include a parity field, a SIG TAIL bit,etc.

The data field may include a service field, a PLCP Service Data Unit(PSDU), and a PPDU TAIL bit. If necessary, the data field may furtherinclude a padding bit. Some bits of the SERVICE field may be used tosynchronize a descrambler of the receiver. PSDU may correspond to a MACPDU defined in the MAC layer, and may include data generated/used in ahigher layer. A PPDU TAIL bit may allow the encoder to return to a stateof zero (0). The padding bit may be used to adjust the length of a datafield according to a predetermined unit.

MAC PDU may be defined according to various MAC frame formats, and thebasic MAC frame is composed of a MAC header, a frame body, and a FrameCheck Sequence. The MAC frame is composed of MAC PDUs, such that it canbe transmitted/received through PSDU of a data part of the PPDU frameformat.

A MAC header may include a frame control field, a Duration/ID field, anaddress field, etc. The frame control field may include controlinformation requisite for frame transmission/reception. The Duration/IDfield may be established as a specific time for transmitting thecorresponding frame or the like. For a detailed description of SequenceControl, QoS Control, and HT Control sub-fields of the MAC headerreference may be made to the IEEE 802.11-2012 standard documentation.

The frame control field of the MAC header may include Protocol Version,Type, Subtype, To DS, From DS, More Fragment, Retry, Power Management,More Data, Protected Frame, and Order sub-fields. For a detaileddescription of individual sub-fields of the frame control field may bemade to the IEEE 802.11-2012 standard documentation.

On the other hand, a null-data packet (NDP) frame format may indicate aframe format having no data packet. That is, the NDP frame includes aPLCP header part (i.e., STF, LTF, and SIG fields) of a general PPDUformat, whereas it does not include the remaining parts (i.e., the datafield). The NDP frame may be referred to as a short frame format.

APSD Mechanism

An Access Point (AP) supporting Automatic Power Save Delivery (APSD) mayperform signaling of information indicating that the AP supports APSDusing an APSD subfield contained in a capability information field suchas a beacon frame, a probe response frame, or associated response frame(or re-associated response frame). The STA capable of supporting APSDmay indicate whether to operate in the active mode or in the PS modeusing the power management field contained in the FC field of the frame.

The APSD is a mechanism in which the STA operating in the PS mode cantransmit DL data and a bufferable management frame. A power managementbit of the FC bit of a frame transmitted by the STA operating in the PSmode employing the APSD is set to 1, such that AP buffering may betriggered.

The APSD defines two delivery mechanisms, i.e., Unscheduled-APSD(U-APSD) and Scheduled-APSD (S-APSD). The STA may use the U-APSD in sucha manner that all or some parts of a Bufferable Unit (BU) can betransferred during an unscheduled service period (SP). In addition, theSTA may use the S-APSD to deliver some or all parts of the BU during thescheduled SP.

In accordance with the U-APSD mechanism, the STA may inform the AP of arequested transmission duration so as to use U-APSD SP, and the AP maytransmit a frame to the STA during the SP. In accordance with the U-APSDmechanism, the SSTA may simultaneously receive several PDSUs from the APusing its own SP.

The STA can recognize the presence of data to be received from the APthrough a TIM element of a beacon. Thereafter, the STA transmits atrigger frame to the AP at a desired time so as to inform the AP of thebeginning of STA's SP, such that the STA may transmit a datatransmission request to the AP. The AP may transmit ACK as a response tothe trigger frame. Thereafter, the AP transmits an RTS to the STAthrough competition, receives a CTS frame from the STA, and transmitsdata to the STA. In this case, data transferred from the AP may becomprised of one or more data frames. When the AP transmits the lastdata frame, End Of Service Period (EOSP) of the corresponding data frameis set to 1 and is then transmitted to the STA, the STA may recognizethe EOSP of 1 and terminate the SP. Therefore, the STA may transmit anACK signal indicating successful data reception to the AP. As describedabove, according to the U-APSD mechanism, the STA may start its own SPat a desired time so as to receive data, and receive multiple dataframes within one SP, such that it can more effectively receive data.

The STA configured to use U-APSD may not receive a frame transmitted bythe AP during the service period (SP) due to interference. Although theAP may not detect interference, the AP may decide that the STA hasincorrectly received the frame. Using U-APSD coexistence capability, theSTA may inform the AP of a requested transmission duration, and may usethe requested transmission duration as an SP for U-APSD. The AP maytransmit the frame during the SP, such that the possibility of receivingthe frame can increase under the condition that the STA receivesinterference. In addition, U-APSD may reduce the possibility that theframe transferred from the AP is not successfully received during theSP.

The STA may transmit an ADDTS (Add Traffic Stream) request frameincluding a coexistence element to the AP. The U-APSD coexistenceelement may include information regarding the requested SP.

The AP may process a requested SP and transmit the ADDTS response frameas a response to the ADDTS request frame. The ADDTS request frame mayinclude a status code. The status code may indicate response informationof the requested SP. The status code may indicate whether or not therequested SP is allowed, and may further indicate a reason of rejectionwhen the requested SP is rejected.

If the requested SP is allowed by the AP, the AP may transmit the frameto the STA during the SP. The duration time of SP may be specified bythe U-APSD coexistence element contained in the ADDTS request frame. Thebeginning point of SP may be a specific time at which the STA transmitsa trigger frame to the AP such that the AP is normally received.

The STA may enter a sleep state (or doze status) when U-APSD SP expires.

EDCA Parameter

EDCA parameter may include necessary information for enabling the STA toproperly operate according to QoS characteristics during a contentionperiod (CP). In order to establish or change a policy by the AP when anew STA is received or a new traffic starts, the EDCA parameter set IEmay be applied to the STA. The STA may update an appropriate ManagementInformation Base (MIB) value according to the latest received EDCAparameter set IE.

The EDCA parameter set IE may include an access category (AC) value tobe applied to the STA. The AC may refer to a priority level in EDCA. Inmore detail, a label of the set of parameters configured to performchannel access competition so as to transmit data (for example, MSDU)according to a predetermined priority is referred to as ‘AC’. Forexample, the AC may be classified into AC_BK(Background), AC_BE(BestEffort), AC_VI(Video), AC_VO(Voice), etc. The ascending numerical orderof priorities is AC_BK→AC_BE→AC_VI→AC_VO. That is, AC_BK has the lowestpriority, and AC_VO has the highest priority. Traffic having a high AC(or high priority) may be transmitted at a higher probability ascompared to other traffic having a low AC (or low priority). Forexample, the STA having higher-priority traffic is in the standby modefor a shorter time, on average, so as to transmit many more packets thanthe STA having lower-priority traffic. For example, the STA havinghigher-priority traffic is in the standby mode for a shorter time, onaverage, so as to transmit many more packets as compared to another STAhaving lower-priority traffic.

In addition, the EDCA parameter set IE may include associatedinformation (for example, CWmin and CWmax for specifying the range of acontention window (CW)) regarding a time period to which a specific ACis applied.

PS-Poll Transmission Method

As can be seen from FIGS. 10 and 11, the STA configured to operate onthe basis of a TIM may confirm whether a buffered frame to be receivedby the STA is contained in the AP using a TIM element obtained throughthe beacon frame, etc. The STA may transmit the PS-Poll frame to the APso as to receive the buffered frame, and may transmit the ACK frame orthe buffered frame to the STA in response to the PS-Poll frame.

If a large number of STAs receiving the buffered frame are present, manySTAs may simultaneously transmit the PS-Poll frame. The STA havingconfirmed the TIM of the beacon frame may transmit the PS-Poll framewithin the beacon interval. If the number of STAs transmitting thePS-Poll frame increases during the beacon interval corresponding to agiven time length, there is a high possibility of collision between thePS-Poll frames. On the other hand, if AC of the STA is set to AC_BE, theSTA having AC_BE is in the standby mode for a longer time (i.e., backsoff for a longer time) before frame transmission as compared to otherSTAs having AC_VI or AC_VO, such that frame transmission of many STAsmay reduce the probability of causing overlap between timing points offrame transmission. Accordingly, the PS-Poll frame has been defined tobe transmitted using ‘AC_BE’.

If channel access priority of the PS-Poll frame is set to AC_BE, thepossibility of causing collision can be reduced whereas STA powerconsumption caused by the increased backoff time unavoidably increases.Accordingly, there is a need to increase channel access priority of thePS-Poll frame so as to reduce STA power consumption. However, the legacyEDCA operation cannot separately allocate the channel access priority(AC) other than AC_BE to a control frame such as a PS-Poll frame.

The present invention provides a method for separately (or adaptively)establishing an AC applied to the PS-Poll frame. For example, whileAC_BE is applied to the PS-Poll frame in a general case, a higherchannel access priority (for example, AC_VO) may be used at a lowpossibility of causing collision between PS-Poll frames. The case inwhich the possibility of causing collision between the PS-Poll frames isnot high is classified into two cases. One case may indicate PS-Pollframe transmission of a non-TIM STA, and the other case may indicatethat RAW for TIM STA is established.

Non-TIM STA is an STA capable of operating without receiving a TIM fromthe AP (without receiving a beacon frame from the AP). For example, alow-power STA configured to operate in an application network such as asensor, a smart grid, an M2M, an Internet of things, or the like, mayoperate in the non-TIM mode. It is next to impossible for the STAoperating in a sleep mode for a long period of time to correctlyestimate a beacon transmission time (i.e., target beacon transmissiontime (TBTT)) from the AP due to the influence of clock drift and thelike, such that the STAs need to operate without receiving the beacon.Accordingly, the non-TIM STA awakes from the sleep mode at an arbitrarytime without receiving the beacon, a PS-Poll is sent to the AP (where,the PS-Poll may be referred to as an active Poll distinguished fromanother PS-Poll sent from the legacy TIM STA), such that the non-TIM STAcan confirm the presence or absence of a buffered frame to be receivedby the non-TIM STA. Alternatively, the non-TIM STA awakes at a specifictime (for example, a target wake time (TWT)) decided by consultationwith the AP, and transmits the PS-Poll frame to the AP, such that thenon-TIM STA can recognize the presence or absence of a buffered frame tobe received by the non-TIM STA.

As described above, assuming that transmission of the PS-Poll frame isnot based on information indicated by the TIM element (or PS-Poll frametransmission is not triggered by the TIM element), PS-Poll frametransmission time points of respective STAs may be distributed atrandom. Accordingly, although there are a large number of STAstransmitting the PS-Poll frame, the possibility of collision between thePS-Poll frames is not high in the non-TIM STA. Therefore, assuming thatthe channel access priority of the non-TIM STA is assigned higherpriority (e.g., AC_VO) instead of AC_BE, power consumption of thenon-TIM STA can be greatly reduced. As a result, a default value of theaccess category used for PS-Poll frame transmission of the non-TIM STAmay be set to AC_VO.

If RAW is established in TIM STA, AC for PS-Poll frame transmission ofthe TIM STA may be established. TIM STA may confirm a TIM elementcontained in the beacon frame received from the AP. If the bufferedframe exists, the PS-Poll receiving the buffered frame may betransmitted to the AP. If there are a large number of STAs indicated bythe TIM element indicating the presence of a buffered frame, there is ahigh possibility that STAs can simultaneously transmit the PS-Pollframe. In order to avoid simultaneous transmission of the PS-Pollframes, PS-Poll transmission time points may be distributed through thesame random variable as a bitmap position value of STAs in the TIMelement. For example, the AP may establish a RAW period through thebeacon frame.

FIG. 15 is a conceptual diagram illustrating a Restricted Access Window(RAW) structure according to one example of the present invention.

RAW may be defined as a time period in which access of the STA group isrestrictively allowed. For example, STAs corresponding to an AID of aspecific range may construct a single RAW group. One RAW may bespecified according to a RAW start time and a RAW duration. In addition,RAW may be composed of one or more slots. If one RAW includes aplurality of slots, durations of slots may have the same values. In FIG.15, 6 slots are defined per RAW duration, and durations of the 6 slotsare set to the same value. In FIG. 15, a slot boundary may indicate areference point for discriminating among contiguous slots.

If RAW is established as described above, only STA(s) to which channelaccess is permitted by the AP can perform channel access. STAsconfigured to receive the beacon frame and confirm the TIM element canrecognize the presence or absence of a buffered frame. The STA thatallows the AP to receive the buffered frame can transmit the PS-Pollframe to the AP. If RAW is configured in STAs, PS-Poll frametransmission of a specific STA is allowed only within a specific slot ofthe corresponding RAW. For this purpose, RAW configuration may includeslot assignment information. In this case, the slot assignment uses thesame random variable as the bitmap position value of STAs in the TIMelement, such that there is a relatively low possibility of generatingcollision between PS-Poll frame transmissions of STAs assigned the sameRAW.

Therefore, if the channel access priority (or AC) applied to the PS-Pollframe transmitted from the STA within the RAW period is assigned a highvalue (e.g., AC_VO) higher than the legacy AC_BE, STA power consumptioncan be greatly reduced. Therefore, a default value of the accesscategory used for PS-Poll frame transmission within the RAW period maybe set to AC_VO.

In addition, assuming that the AP desires to change the AC used forPS-Poll frame transmission of the STAs, a method for establishing thePS-Poll access category through a beacon frame, a probe response frame,and an associated response frame (or re-associated response frame) isproposed. For this purpose, a new field contained in the beacon frame orthe like may be defined. For example, 2 bits from among reserved bits ofthe Extended Capability IE are defined as a PS-Poll AC field, and the ACvalue used for PS-Poll frame transmission may be transferred to the STAusing the PS-Poll AC field. Alternatively, instead of using some bits ofthe legacy Extended Capability IE field, a new IE is defined as shown inFIG. 16 and an AC for the PS-Poll frame may be separately applied to theSTA as necessary.

FIG. 16 is a structural diagram illustrating an Information Element (IE)used for establishing the PS-Poll AC according to the present invention.

Referring to FIG. 16(a), Element ID may be assigned a specific valueindicating that the corresponding IF is an IE for the PS-Poll AC. ALength field may be assigned a specific value indicating the length of aPS-Poll AC field. A PS-Poll Access Category field may be assigned an ACvalue to be used for PS-Poll frame transmission.

For example, PS-Poll AC information may be transferred from the AP tothe STA through the beacon frame. Although the non-TIM STA operateswithout confirming a TIM contained in the beacon frame, the PS-Poll ACinformation proposed by the present invention may be transferred fromthe AP to the STA through two steps requisite for recognizing basic APoperation information. In more detail, a first step of the two stepsallows the STA to discover the AP, and a second step of the two stepsallows the STA to associate with the AP. Accordingly, an AC of thePS-Poll frame may be applied to the STA according to the AC valueestablished by the AP. For example, the AC value used for PS-Polltransmission of the non-TIM STA may be set to AC_VO through the PS-PollAC field contained in the beacon frame. Alternatively, if the AC valueapplied to PS-Poll transmission of the non-TIM STA is set to a defaultAC_VO, a different value may be assigned to the PS-Poll AC.

In addition, the AC value used for PS-Poll frame transmission isindicated, such that the corresponding AC may provide a time interval orscheduling information to be applied to PS-Poll frame transmission. Forexample, FIG. 16(b) may further include a duration field and an intervalfield as compared to FIG. 16(a). The duration field may be set to aspecific value indicating a time duration to Which the PS-Poll ACestablished by AP is applied. That is, the PS-Poll frame transmittedduring a specific time corresponding to a specific value indicated bythe duration field may be based on priority of the AC value establishedby the PS-Poll AC field. The interval field may be set to a specificvalue indicating a time interval in which the AC used for PS-Poll frametransmission is changed. For example, the PS-Poll AC value establishedby the AP may be applied to a predetermined time duration repeated atintervals of a predetermined time.

PS-Poll Response Method

Long sleeper STAs may refer to STAs capable of maintaining the sleepmode (or doze mode) for a long period of time. Long sleeper STAs awakeat a predetermined time (e.g., target wake time (TWT)) allocated by theAP, and transmit the PS-Poll frame. Alternatively, the long sleeper STAsare synchronized with the AP upon receiving the beacon frame, and canconfirm the presence or absence of a buffered frame to be received. Ifthe sleep duration of the long sleeper STAs is elongated, an error of alocal clock (or local timer) of the STA increases, such that it isdifficult to correctly estimate a TBTT (Target Beacon Transmit Time).Therefore, long sleeper STAs awake at an arbitrary time point, transmita short control frame such as the PS-Poll frame to the AP so as toestablish synchronization with the AP, and confirm the presence orabsence of a buffered frame to be received by each long sleeper STA,resulting in higher efficiency in terms of energy consumption.

The non-TIM STA from among long sleeper STAs is restricted to an STAthat does not receive signaling information (e.g., a TIM element)indicating a buffered frame from the AP. That is, the non-TIM STA mayoperate without receiving the beacon frame from the AP. If a TimingSynchronization Function (TSF) error between the STA and the AP occursfor 100 ms or longer, it is next to impossible for the corresponding STAto estimate a beacon transmission time (i.e., TBTT) of the AP. Insteadof reception of the beacon frame, non-TIM STAs awake at an arbitrarytime and transmit the PS-Poll frame to the AP, such that each non-TIMSTA may recognize the presence or absence of a buffered frame to bereceived by the non-TIM STA.

If the non-TIM STAs awake at an arbitrary time and transmit the PS-Poll,trigger frame, or UL data frame, the corresponding transmission starttime overlaps with those of legacy communication standards, resulting inthe occurrence of interference or collision.

FIG. 17 is a conceptual diagram illustrating a PS-Poll response methodaccording to one example of the present invention.

Referring to FIG. 17, it is assumed that STA1 is denoted by a non-TIMSTA, and BSS1 of AP1 is contiguous to or overlaps with BSS2 of AP2. Inaddition, STA1 is associated with AP1 and STA2 is associated with AP2.In addition, it is assumed that the relationship between STA1 and STA2is a hidden node. That is, a hidden node of STA1 is an STA2, and ahidden node of STA2 is an STA1, such that STA1 cannot listen totransmission information of the STA2, and STA2 cannot listen totransmission information of the STA1.

In FIG. 17(a), we assume that, before the non-TIM STA (i.e., STA1)awakes, another STA (i.e., STA2) acquires a transmission opportunity(TXOP) through channel access. In this case, STA2 may belong to a BSS(OBSS) overlapped with a BSS of STA1, or may perform communicationbetween one STA and another STA (i.e., STA3) (where, AP2 of FIG. 17 maybe replaced with STA3). When STA2 establishes the TXOP, a time periodcorresponding to a NAV of peripheral AP(s) (including AP1 connected tothe non-TIM STA) may be set to TXOP duration through the RTS/CTS processassociated with AP2.

If STA1 is a hidden node incapable of listening to communicationinformation of STA2, STA1 cannot recognize channel access of STA2, sothat it determines an idle status of a channel and transmits the PS-Pollframe to AP1. That is, after STA1 performs backoff through EDCA, STA1may transmit the PS-Poll frame to the AP. If the AP having received thePS-Poll frame answers the ACK frame without confirming its own channelstate, the corresponding ACK frame may cause interference or collisionto legacy communication (e.g., communication between STA2 and AP2).

In order to solve the above-mentioned problem, the present inventionprovides a method for enabling the AP having received the PS-Poll frameto check a channel state thereof before transmitting a response frame(e.g., ACK frame or data frame) to the STA.

The example of FIG. 17(b) assumes the presence of a peripheral situationsimilar to FIG. 17(a). That is, if STA1 incapable of listening tocommunication between STA2 and AP2 transmits the PS-Poll frame to AP1,the AP1 may confirm a channel state thereof before transmitting a frameanswering the PS-Poll frame of the STA1. The AP1 in which NAV isestablished by channel access between STA2 and AP2 can recognize that achannel is occupied during the NAV period, such that the AP1 need nottransmit a response frame to STA1. Accordingly, channel access ofanother STA can be protected.

In accordance with the proposal of the present invention, if the APreceives the PS-Poll frame from the STA, the AP can operate as followsin consideration of the non-TIM STAs.

If the AP receives the PS-Poll frame from the STA, the AP may determinewhether an AID value of the STA contained in the PS-Poll frame belongsto the Aid range allocated to the non-TIM STAs.

If the PS-Poll frame receives the non-TIM STA, the AP may recognize achannel state thereof before the ACK frame or the buffered DATA frame istransmitted to the corresponding STA.

If NAV is not established (or if a channel is in an idle mode) afterreception of the PS-Poll frame, the AP may transmit the ACK frame or thebuffered frame to the STA.

If the AP always performs physical carrier sensing, the AP may generatea response signal using the ACK frame or the buffered DATA frameaccording to the physical carrier sense result obtained before receptionof the PS-Poll frame. That is, if the physical carrier sense result of aprimary channel is in the idle state prior to reception of the PS-Pollframe, the AP may transmit the ACK frame or the buffered DATA frame tothe STA.

Information as to which one of the ACK frame or the buffered DATA framewill be used for such answering may be changed according to theimplementation scheme of an AP processing throughput, etc. If thebuffered DATA frame can be immediately transmitted within apredetermined time (e.g., SIFS) after the AP receives the PS-Poll frame,the AP may generate a response signal using the buffered DATA frame.Alternatively, if the AP cannot transmit the buffered DATA frame withina predetermined time, the AP may generate a response signal using theACK frame.

On the other hand, if the AID value of the STA contained in the PS-Pollframe is not contained within the AID range allocated to the non-TIMSTAs, the AP need not confirm a channel status thereof when generating aresponse signal using the ACK frame or the buffered DATA frame.

That is, if a general STA (e.g., TIM STA) instead of the non-TIM STAtransmits the PS-Poll frame, the AP having successfully received thePS-Poll frame may always transmit the ACK frame or the buffered DATAframe after lapse of SIFS, irrespective of a channel status thereof.

As described above, if the non-TIM STA awakes from the sleep mode andtransmits the PS-Poll frame, there is a high possibility that thenon-TIM STA is not synchronized with the AP. Accordingly, after thenon-TIM STA confirms a channel status when answering the PS-Poll frame,the AP transmits the ACK frame or the buffered data frame, such thatchannel access of another STA can be protected.

For example, an STA that is addressed by an PS-Poll frame shall transmiteither an ACK frame or Buffered DATA/Management frame after an SIFSperiod if the NAV at the STA receiving the PS-Poll frame indicates thatthe medium is idle. If the NAV at the STA receiving the PS-Poll frameindicates the medium is not idle, that STA shall not respond to PS-Pollframe.

FIG. 18 is a conceptual diagram illustrating a PS-Poll response methodaccording to another example of the present invention.

Referring to FIG. 18, the AP having received the PS-Poll frame (from thenon-TIM STA) may determine whether a channel state checking (e.g., NAVconfirmation) operation is carried out according to response types.

In the case of the NDP ACK frame, message information associated withthe ACK frame is not contained in a PDSU and is contained in a PHY layerfield (e.g., PLCP header part (e.g., STF, LTF, and SIG fields). If theND PACK frame is transmitted as a response frame of the PS-Poll frame,the AP does not confirm a channel state before transmitting a responseto the PS-Poll frame, because the NDP ACK frame has a very short lengthcorresponding to the length of several OFDM symbols such that a time orinfluence causing interference to other STA/AP communication is veryshort.

The NDP ACK frame is comprised of only STF, LTF and SIG fields. Achannel estimation sequence for decoding the SIG field may be containedin the STF and LTF fields. Message information associated with the ACKframe may be contained in the SIG field. ACK-frame associated messageinformation contained in the SIG field may include a More Data bitindicating whether or not a. Receiver Address and a buffered frame to bereceived by the corresponding STA are contained in the AP.

The AP need not confirm channel states of a plurality of frames beforeanswering the PS-Poll frame. For example, the plurality of frames mayinclude an NDP ACK frame, a normal ACK frame, an RTS frame, a CTS frame,etc.

That is, assuming that the AP receives the PS-Poll frame and is ready totransmit not only the NDP ACK frame as a response to the PS-Poll frame,but also control frames (such as normal ACK frame, RTS frame, and CTSframe), the AP may transmit the corresponding response frame withoutconfirming whether its own channel state (e.g., NAV value) is a busy oridle state.

Alternatively, assuming that the buffered DATA frame used as a responseto the PS-Poll frame is transmitted to the AP, the AP confirms its ownchannel state (e.g., NAV value) before transmitting the DATA frame, suchthat it can transmit a response frame only in the idle state.

Before the AP receives the PS-Poll frame and transmits a response frame,the AP may recognize its own channel state (e.g., NAV value) using somekinds of frame types. For example, the above frame types may berestricted to a DATA frame, a management frame, etc.

Alternatively, it may be determined whether the AP confirms a channelstatus before transmission of a response frame on the basis of atransmission (Tx) time of a response frame or the size (e.g., the numberof bytes or the number of octets) of the response frame.

If the AP receives the PS-Poll frame and the Tx time or size of aresponse frame used as a response to the PS-Poll frame is equal to orhigher than a predetermined threshold value, the AP may confirm its ownchannel state (e.g., NAV value) before transmitting the correspondingresponse frame. Alternatively, if the Tx time or size of the responseframe is less than the predetermined threshold value, the AP may notconfirm its own channel status before transmitting the correspondingresponse frame.

FIG. 18(a) shows that AP1 having received the PS-Poll frame from STA1(i.e., non-TIM STA) desires to generate a response signal using the ACKframe. If the response frame type is an ACK frame, AP1 may transmit theACK frame without confirming the NAV value. Alternatively, AP1 confirmsits own NAV value, such that it can transmit the ACK frame even though achannel (or medium) is in a busy state due to communication between STA2and AP2.

FIG. 18(b) shows that the AP1 having received the PS-Poll frame fromSTA1 (i.e., non-TIM STA) desires to generate a response signal using thebuffered DATA frame. If the response frame type is a DATA frame, AP1confirms its own NAV value, such that the AP1 may transmit a responseframe (e.g., DATA frame) only when a channel (or medium) is in an idlemode. The example of FIG. 18(b) shows that a response frame type is aDATA frame and the NAV value of the AP1 indicates a channel busy state,such that AP1 may not transmit the DATA frame.

As described above, assuming that the AP may or may not confirm thechannel status according to response frame types, if the AP receives thePS-Poll frame and a channel state of the AP is not idle, only thedeferred PS-Poll procedure (i.e., the ACK frame is first transmitted inresponse to the PS-Poll frame and the DATA frame is then transmitted)instead of the immediate PS-Poll procedure (i.e., the DATA frame isimmediately transmitted in response to the PS-Poll frame) may beselected as necessary.

In other words, if the AP channel state is not idle (e.g., if the NAVvalue indicates a channel busy state), a response frame of the PS-Pollframe may not be used as the DATA frame. In this case, the AP firsttransmits the ACK frame instead of the DATA frame, informs the STA ofwhether the PS-Poll frame has been successfully received by the AP, andat the same time informs the STA of whether the DATA frame to bereceived by the corresponding STA is buffered by the AP.

The AP channel state is not in the idle mode. Assuming that a responseto the PS-Poll frame is used for the ACK frame but not the DATA frame,the corresponding ACK frame may further include information of a timingpoint at which the AP can deliver the DATA frame. The timing informationmay be denoted by a time offset value.

For example, the AP can immediately generate a response signal using theDATA frame upon receiving the PS-Poll frame from the STA. However,assuming that the AP first transmits the ACK frame under a channel busystate, the ACK frame may include a duration value of a NAV establishedin the AP or other values in the time offset value.

The non-TIM STA having received the ACK frame including the time offsetvalue as a response to the PS-Poll frame operates in the sleep modeduring a time offset, awakes from the sleep mode at a specific timespecified by the time offset, and then receives the buffered frame fromthe AP.

FIG. 19 is a flowchart illustrating the PS-Poll process according to oneexample of the present invention.

Referring to FIG. 19, an STA may receive PS-Poll access category (AC)information from an AP in step S1910. The PS-Poll AC field may beprovided to the STA through a beacon frame.

The STA may transmit the PS-Poll frame to the AP in step S1920. In stepS1920, the STA may transmit the PS-Poll frame using the PS-Poll AC shownin step S1910. In this case, the STA does not confirm a TIM providedthrough the beacon frame, and may be a non-TIM STA configured totransmit the PS-Poll frame at a time point at which the STA awakes. Inthis case, a time point (or a specific time at which a non-TIM STAawakes) at which the non-TIM STA transmits the PS-Poll frame may beallocated or established by the AP.

In step S1930, the STA may receive a response frame (e.g., ACK frame,buffered DATA frame, etc.) to the PS-Poll frame from the AP. In thiscase, the AP may confirm a channel status (or its own NAV value) beforetransmitting a response frame of the PS-Poll frame. Alternatively,whether to perform the operation for confirming the channel state may bedecided according to response frame types of the PS-Poll frame.

PS-Poll transmission/reception and the method for transmitting andreceiving the PS-Poll response frame according to the embodiment shownin FIG. 19 may be implemented such that the above described variousembodiments of the present invention may be independently applied or twoor more embodiments thereof may be simultaneously applied.

FIG. 20 is a block diagram illustrating a radio frequency (RF) deviceaccording to one embodiment of the present invention.

Referring to FIG. 20, an AP 10 may include a processor 11, a memory 12,and a transceiver 13. An STA 20 may include a processor 21, a memory 22,and a transceiver 23. The transceivers 13 and 23 may transmit/receiveradio frequency (RF) signals and may implement a physical layeraccording to an IEEE 802 system. The processors 11 and 21 are connectedto the transceivers 13 and 21, respectively, and may implement aphysical layer and/or a MAC layer according to the IEEE 802 system. Theprocessors 11 and 21 may be configured to operate according to the abovedescribed various embodiments of the present invention. Modules forimplementing operation of the AP and STA according to the abovedescribed various embodiments of the present invention are stored in thememories 12 and 22 and may be implemented by the processors 11 and 21.The memories 12 and 22 may be included in the processors 11 and 21 ormay be installed at the exterior of the processors 11 and 21 to beconnected by a known means to the processors 11 and 21.

The AP 10 shown in FIG. 10 may support frame transmission (in moredetail, PS-Poll frame transmission) of the STA 20. The processor 11awakes at a predetermined time, and enables the AP 10 to receive thePS-Poll frame from the STA 20 using the transceiver 13. In addition, theprocessor 11 may enable the AP 10 to transmit response information ofthe PS-Poll frame to the STA 20 using the transceiver 13. Here, specificinformation indicating the access category (AC) of the PS-Poll frame istransferred from the AP 10 to the STA 20, and the STA 20 may transmitthe PS-Poll according to the indicated PS-Poll access category (AC).

The STA 20 shown in FIG. 20 may be configured to perform frametransmission (in more detail, PS-Poll frame transmission). The processor21 awakes at a predetermined time, and enables the STA 20 to transmitthe PS-Poll frame to the AP 10 using the transceiver 23. In addition,the processor 11 may enable the AP 10 to transmit response informationof the PS-Poll frame to the STA 20 using the transceiver 13. Inaddition, the processor 21 may enable the STA 20 to receive responseinformation of the PS-Poll frame from the STA 20 using the transceiver23. Here, specific information indicating the access category (AC) ofthe PS-Poll frame is transferred from the AP 10 to the STA 20, and theSTA 20 may transmit the PS-Poll frame according to the indicated PS-Pollaccess category (AC).

In addition, the AP 10 may confirm a channel status before a response tothe PS-Poll is transferred from the STA 20, may determine whether toconfirm a channel status according to response types, and may operateaccording to the determined result.

The overall configuration of the AP 10 and the STA 20 shown in FIG. 20may be implemented such that the above described various embodiments ofthe present invention may be independently applied or two or moreembodiments thereof may be simultaneously applied and a repeateddescription thereof is omitted for clarity.

The above-described embodiments may be implemented by various means, forexample, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments ofthe present invention may be implemented by one or more ApplicationSpecific Integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to theembodiments of the present invention may be implemented in the form ofmodules, procedures, functions, etc. performing the above-describedfunctions or operations. Software code may be stored in a memory unitand executed by a processor. The memory unit may be located at theinterior or exterior of the processor and may transmit and receive datato and from the processor via various known means.

The detailed description of the preferred embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the preferred embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. Accordingly, the inventionshould not be limited to the specific embodiments described herein, butshould be accorded the broadest scope consistent with the principles andnovel features disclosed herein.

INDUSTRIAL APPLICABILITY

Although the above various embodiments of the present invention havebeen described based upon an IEEE 802.11 system, the embodiments may beapplied in the same manner to various mobile communication systems.

The invention claimed is:
 1. A method for transmitting a frame by afirst type station (STA) in a wireless LAN (WLAN) system, the methodcomprising: receiving an information element from an access point (AP),wherein the information element includes a Power Save (PS)-Poll AccessCategory (AC) field indicating an AC of a PS-Poll frame to betransmitted by the first type STA, wherein the PS-Poll AC field of theinformation element is reserved for a second type STA, wherein the firsttype STA is a STA to which at least one of a non-TIM (Traffic IndicationMap) operation and a RAW (Restricted Access Window) operation can beapplied, wherein the second type STA is a STA to which the non-TIMoperation and the RAW operation cannot be applied, wherein the non-TIMoperation is an operation for the first type STA to: not listen for aTIM element from the AP, and transmit a PS-Poll frame to the AP, whereinthe RAW operation is an operation for the AP to allocate a RAW to alimited number of the first type STAs to spread an uplink access of thefirst type STA, wherein information on the PS-Poll AC field can indicatean AC other than an Access Category-Best Effort (AC_BE) while atransmission of a PS-Poll frame by the second type STA is predeterminedas using only the AC_BE to reduce collision probability; transmitting,by the first type STA, the PS-Poll frame to the AP according toinformation on the PS-Poll AC field; and receiving a response inresponse to the PS-Poll frame from the AP by the first type STA.
 2. Themethod according to claim 1, wherein the information on the PS-Poll ACfield includes an AC field applied to the PS-Poll frame to betransmitted within the RAW.
 3. The method according to claim 2, whereinthe AC field is used for the AP to inform the first type STA of anaccess category for transmitting the PS-Poll frame in the RAW.
 4. Themethod according to claim 1, wherein the information element is includedin a beacon frame.
 5. The method according to claim 1, wherein thePS-Poll AC field is two bits long.
 6. The method according to claim 1,wherein the transmission of the PS-Poll frame by the first type STA isperformed with a backoff procedure for accessing a wireless medium inthe RAW using the access category corresponding to a specific value setby the PS-Poll AC field.
 7. The method according to claim 1, wherein thefirst type STA is a non-TIM STA.
 8. A method for supporting transmissionof a first type station (STA) by an access point (AP) in a wireless LAN(WLAN) system, the method comprising: transmitting an informationelement by the AP, wherein the information element includes a Power Save(PS)-Poll Access Category (AC) field indicating an AC of a PS-Poll frameto be transmitted by the first type STA, wherein the PS-Poll AC field ofthe information element is reserved for a second type STA, wherein thefirst type STA is a STA to which at least one of a non-TIM (TrafficIndication Map) operation and a RAW (Restricted Access Window) operationcan be applied, wherein the second type STA is a STA to which thenon-TIM operation and the RAW operation cannot be applied, wherein thenon-TIM operation is an operation for the first type STA to: not listenfor a TIM element from the AP, and transmit a PS-Poll frame to the AP,wherein the RAW operation is an operation for the AP to allocate a RAWto a limited number of the first type STAs to spread an uplink access ofthe first type STA, wherein information on the PS-Poll AC field canindicate an AC other than an Access Category-Best Effort (AC_BE) while atransmission of a PS-Poll frame by the second type STA is predeterminedas using only the AC_BE to reduce collision probability; receiving thePS-Poll frame from the first type STA according to information on thePS-Poll AC field; and transmitting a response in response to the PS-Pollframe to the first type STA, wherein information indicating an AccessCategory (AC) of the PS-Poll frame to be transmitted by the first typeSTA within the RAW is provided from the AP to the first type STA, thefirst type STA being a STA of which transmission is restricted to aRestricted Access Window (RAW) allocated by the AP.
 9. A first typestation (STA) device for transmitting a frame in a wireless LAN (WLAN)system, the first type STA comprising: a transceiver; and a processorconfigured to: receive, using the transceiver, an information elementfrom an access point (AP); transmit, using the transceiver, a Power Save(PS)-Poll frame to an access point (AP); and receive, using thetransceiver, a response in response to the PS-Poll frame from the AP,wherein the information element includes a PS-Poll Access Category (AC)field indicating an AC of the PS-Poll frame to be transmitted by thefirst type STA, wherein the PS-Poll AC field of the information elementis reserved for a second type STA, wherein the first type STA is a STAto which at least one of a non-TIM (Traffic Indication Map) operationand a RAW (Restricted Access Window) operation can be applied, whereinthe second type STA is a STA to which the non-TIM operation and the RAWoperation cannot be applied, wherein the non-TIM operation is anoperation for the first type STA to: not listen for a TIM element fromthe AP, and transmit a PS-Poll frame to the AP, wherein the RAWoperation is an operation for the AP to allocate a RAW to a limitednumber of the first type STAs to spread an uplink access of the firsttype STA, wherein information on the PS-Poll AC field can indicate an ACother than an Access Category-Best Effort (AC_BE) while a transmissionof a PS-Poll frame by the second type STA is predetermined as using onlythe AC_BE to reduce collision probability.
 10. An access point (AP)device for supporting transmission of a first type station (STA) in awireless LAN (WLAN) system, the AP comprising: a transceiver; and aprocessor configured to: transmit, using the transceiver, an informationelement; receive, using the transceiver, a Power Save (PS)-Poll framefrom the first type STA; and transmit, using the transceiver, a responseinformation in response to the PS-Poll frame to the first type STA,wherein the information element includes a PS-Poll Access Category (AC)field indicating an AC of a PS-Poll frame to be transmitted by the firsttype STA, wherein the PS-Poll AC field of the information element isreserved for a second type STA, wherein the first type STA is a STA towhich at least one of a non-TIM (Traffic Indication Map) operation and aRAW (Restricted Access Window) operation can be applied, wherein thesecond type STA is a STA to which the non-TIM operation and the RAWoperation cannot be applied, wherein the non-TIM operation is anoperation for the first type STA to: not listen for a TIM element fromthe AP, and transmit a PS-Poll frame to the AP, wherein the RAWoperation is an operation for the AP to allocate a RAW to a limitednumber of the first type STAs to spread an uplink access of the firsttype STA, wherein information on the PS-Poll AC field can indicate an ACother than an Access Category-Best Effort (AC_BE) while a transmissionof a PS-Poll frame by the second type STA is predetermined as using onlythe AC_BE to reduce collision probability.